Semiconductor device

ABSTRACT

Semiconductor devices and fabrication methods are provided. A semiconductor device includes a substrate, a source and drain material layer formed on the substrate. The source and drain material layer contains a first trench there-through. The semiconductor device further includes a mask layer formed on the source and drain material layer containing a second trench there-through. The second trench has a cross-section area larger than the first trench and covers the first trench. The semiconductor device further includes a channel material layer conformally formed on a bottom and sidewalls of each of the first trench and the second trench and a gate structure conformally formed on the channel material layer, on the bottom and the sidewalls of each of the first trench and the second trench. The gate structure has a recess and the recess has a symmetrical step structure.

CROSS-REFERENCES TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 15/964,738, filed on Apr. 27, 2018, which claimsthe priority of Chinese Patent Application No. 201710310410.X, filed onMay 5, 2017, the content of which is incorporated by reference in itsentirety.

TECHNICAL FIELD

The present disclosure generally relates to the field of semiconductortechnology and, more particularly, relates to semiconductor devices andfabrication methods of the semiconductor devices.

BACKGROUND

The MOS (metal-oxide-semiconductor) transistor is one of the mostimportant elements in modern integrated circuits. The basic structure ofthe MOS transistor includes a semiconductor substrate, and a gatestructure located on the surface of the semiconductor substrate. And thegate structure includes: a gate dielectric layer located on the surfaceof the semiconductor substrate; a gate electrode layer located on thesurface of the gate dielectric layer; and a source and drain dopedregion located in the semiconductor substrate at the two sidewalls ofthe gate structure.

With the development of semiconductor technology, the control capabilityof the traditional planar MOS transistor on the channel current isweakened, and serious leakage current is produced as a result. The finfield effect transistor is an emerging multi-gate device, thesemiconductor substrate generally includes a fin structure whichprotrudes out of the surface of the semiconductor substrate, a gatestructure covering the top surface and the sidewalls of the fin, and asource and drain doped region located in the fin portions on the twosides of the gate structure.

With the further development of the semiconductor technology, there arehigher requirements for control capability of the channel current fornot only the planar MOS transistor but also the fin field effecttransistor. For this reason, a semiconductor device with an ultra-thinchannel was used. However, the performance of conventional semiconductordevices having an ultra-thin channel still needs to be improved. Thedisclosed devices and methods are directed to at least partiallyalleviate one or more problems set forth above and to solve otherproblems in the art.

SUMMARY

One aspect of the present disclosure provides a method for forming asemiconductor device. The method includes: providing a substrate;forming a source and drain material layer over the substrate; forming amask layer on the source and drain material layer, and the mask layerincludes a first trench and the first trench exposes a portion of thesource and drain material layer; forming a protective layer on sidewallsof the first trench; forming a second trench in the source and drainmaterial layer by etching the source and drain material layer at abottom of the first trench using the mask layer and the protective layeras a mask; removing the protective layer after the second trench isformed; forming a channel material layer and a gate structure on thechannel material layer after the protective layer is removed, and thechannel material layer is on the sidewalls and the bottom of the firsttrench and on sidewalls and bottom of the second trench; and removingthe mask layer after the channel material layer and the gate structureare formed.

Another aspect of the present disclosure provides a semiconductordevice. The semiconductor device includes: a substrate; a source anddrain material layer formed on the substrate, the source and drainmaterial layer containing a first trench there-through; a mask layerformed on the source and drain material layer containing a second trenchthere-through, the second trench having a cross-section area larger thanthe first trench and covers the first trench; a channel material layerconformally formed on a bottom and sidewalls of each of the first trenchand the second trench; and a gate structure conformally formed on thechannel material layer, on the bottom and the sidewalls of each of thefirst trench and the second trench.

Other aspects or embodiments of the present disclosure can be understoodby those skilled in the art in light of the description, the claims, andthe drawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are merely examples for illustrative purposesaccording to various disclosed embodiments and are not intended to limitthe scope of the present disclosure.

FIG. 1 and FIG. 2 are schematic structural diagrams illustrating aprocess of forming a semiconductor device;

FIG. 3 illustrates an exemplary method for forming a semiconductordevice according to the disclosed embodiments; and

FIGS. 4-13 illustrate semiconductor structures corresponding to certainstages of an exemplary fabrication process of a transistor consistentwith the disclosed embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thedisclosure, which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

FIG. 1 and FIG. 2 are schematic structural diagrams illustrating aprocess of forming a semiconductor device.

In FIG. 1, a substrate 100 is provided, and an oxide layer 110 is formedon the substrate 100. A source and drain material layer 120 is formed onthe oxide layer 110, and the source and drain material layer 120 isprovided with a trench 130 passing through the source-drain materiallayer 120.

In FIG. 2, a channel material film (not shown) is formed on the twosidewalls and the bottom portions of trench 130 (refer to FIG. 1), thesource drain material layer 120, and the substrate 100. The material ofthe channel material film may be MoS₂. A gate structure material layermay be formed on the surface of the channel material film. A patternedgate structure material layer and the channel material film form achannel material layer 140 and a gate structure 150 on the top ofchannel material layer 140. The gate structure 150 corresponds to thegate structure material layer.

After the channel material layer 140 and the gate structure 150 areformed, the method further includes the following step: the source anddrain ions are doped in the source and drain material layer 120 on thetwo sides of the gate structure 150; and a source and drain doped regionis formed in the source and drain material layer 120 on the two sides ofthe gate structure 150.

However, the performance of the semiconductor device formed by the abovemethod needs to be improved. The channel material layer 140 is locatednot only on the sidewalls and the bottom of the trench 130, but also onthe source and drain material layer 120 on the two sides of the trench130. As a result, the effective length of the channel material layer 140is relatively large, and distance between the source and drain dopedregions on the two sides of the gate structure 150 is also relativelylarge.

The method for forming the patterned gate structure material layer andthe channel material film includes: forming a patterned photoresistlayer on the gate structure material layer, and the photoresist layeronly covers the gate structure material layer in the trench 130 and theportion of gate structure material layer located on the source and drainmaterial layer 120 on the two sides of the trench 130; and etching thegate structure material layer and the channel material film by using thepatterned photoresist layer as a mask until the source and drainmaterial layer 120 is exposed.

The trench 130 may have a first sidewall and a second sidewall, facingeach other. The first sidewall and the second sidewall of the trench 130are respectively on the source and drain material layer 120.

Due to the influence of the photo etching alignment deviation, theposition of the projection of the center of the gate structure 150 onthe surface of the substrate 100 may be offset from the position of theprojection of the center of the trench 130 on the surface of thesubstrate 100. The position of the projection of the center of thechannel material layer 140 on the surface of the substrate 100 may alsobe offset from the position of the projection of the center of thetrench 130 on the surface of the substrate 100. The size differencebetween the channel material layer 140 on the first side of the trench130 and the second side of the trench 130 is relative large, and thegate structure 150 on the first side of the trench 130 and the gatestructure 150 on the second side of the trench 130 are also different insize. Thus, the size of the source and drain doped region formed in thesource and drain material layer 120 are very different from each otheron the two sides of the trench, and the parameters of the current andthe process design in the source-drain doped region. As a result, thestability of the electrical properties of the semiconductor device maybe lowered.

The present disclosure provides a semiconductor device and a method forforming the semiconductor device. The method includes: providing asubstrate; forming a source and drain material layer over the substrate;forming a mask layer on the source and drain material layer, and themask layer includes a first trench and the first trench exposes aportion of the source and drain material layer; forming a protectivelayer on sidewalls of the first trench; forming a second trench in thesource and drain material layer by etching the source and drain materiallayer at a bottom of the first trench using the mask layer and theprotective layer as a mask; removing the protective layer after thesecond trench is formed; forming a channel material layer and a gatestructure on the channel material layer after the protective layer isremoved, and the channel material layer is on the sidewalls and thebottom of the first trench and on sidewalls and bottom of the secondtrench; and removing the mask layer after the channel material layer andthe gate structure are formed.

With this method, the sizes of the source drain material layers on thetwo sides of the gate structure are consistent, and the stability of theelectrical performance of the semiconductor device may be improved.

In accordance with some embodiments of the present disclosure, FIG. 3 isa flowchart illustrating a method for forming a semiconductor device. Atstep S101, a substrate is provided. A source and drain material layer isformed on the substrate at step S102. At step S103, a mask layer, whichincludes a first trench and the first trench exposes a portion of thesource and drain material layer, is formed on the source and drainmaterial layer. A protective layer is formed on sidewalls of the firsttrench at step S104. At step S105, a second trench passing through thesource and drain material layer is formed by etching the source anddrain material layer at a bottom of the first trench using the masklayer and the protective layer as a mask. The protective layer isremoved after the second trench at step S106. At step S107, a channelmaterial layer and a gate structure are formed on the channel materiallayer after the protective layer is removed, and the channel materiallayer is located on the sidewalls and the bottom of the first trench andthe second trench. And at step 108, the mask layer is removed after thechannel material layer and the gate structure are formed.

FIGS. 4-13 illustrate semiconductor structures corresponding to certainstages of an exemplary fabrication process of a transistor consistentwith the disclosed embodiments.

As shown in FIG. 4, a substrate 200 is provided.

The substrate 200 may be a bulk silicon substrate, a bulk germaniumsubstrate, a glass substrate, a silicon-on-insulator substrate, or agermanium-on-insulator substrate. In one embodiment, the substrate 200may be a bulk silicon substrate.

In one embodiment, an interface layer 210 may be formed on the surfaceof the substrate 200.

The material of the interface layer 210 may be silicon oxide.

The interface layer 210 may isolate the substrate 200 from asubsequently formed source and drain material layer.

In FIG. 5, a source and drain material layer 220 may be formed on thesubstrate 200 and a mask layer 230 may be formed on the source and drainmaterial layer 220, the mask layer 230 may include a first trench 231,and a portion of the source and drain material layer 220 may be exposedin the first trench 231.

In one embodiment, the interface layer 210 may be formed on the surfaceof the substrate 200 before the source and drain material layer 220 isformed.

The material of the source and drain material layer 220 may be, forexample, polycrystalline silicon.

The process for forming the source and drain material layer 220 may be adeposition process, such as a plasma chemical vapor deposition process,a low-pressure chemical vapor deposition process, or a sub-atmosphericpressure chemical vapor deposition process.

The method for forming a mask layer 230 includes: forming a maskmaterial layer on the source and drain material layer 220; and formingthe mask layer 230 by patterning the mask material layer. The mask layer230 may include the first trench 231 passing through the mask layer 230.

The mask layer 230 may be made of, for example, silicon oxide.

In FIG. 6, a protective layer 240 may be formed on the sidewalls of thefirst trench 231.

The mask layer 230 and the protective layer 240 are used jointly todefine the position of a subsequent formed second trench.

The protective layer 240 may be formed by, for example, forming aprotective material layer (not illustrated) on the sidewalls and thebottom of the first trench 231 and on the top surface of the mask layer230; and forming the protective layer 240 by etching back the protectivematerial layer until the top surface of the mask layer 230 and the topsurface of the source and drain material layer 220 are exposed.

The process for forming the protective material layer may be adeposition process, such as an atomic layer deposition process, a plasmachemical vapor deposition process, a low-pressure chemical vapordeposition process, or a sub-atmospheric chemical vapor depositionprocess.

The material of the protective layer 240 and the material of the masklayer 230 are different. The protective layer 240 and the source anddrain material layer 220 may also be made of different materials.

In the process of etching back the protective material layer, theprotective material layer may have a relatively high etching selectionratio value comparing to the source and drain material layer 220. Theprotective material layer may have a higher etching selection ratiocomparing to the mask layer 230. In the subsequent process of removingthe protective layer 240, the protective layer 240 may have a higheretching selection ratio comparing to the source and drain material layer220.

In one embodiment, the protective layer 240 may be made of siliconnitride.

As shown in FIG. 7, the mask layer 230 and the protective layer 240 areused as a mask to etch the source and drain material layer 220 on thebottom of the first trench 231, and the source and drain material layer220 may include a second trench 222 passing through the source and drainmaterial layer 220.

Using the mask layer 230 and the protective layer 240 as a mask, theprocess of etching the source and drain material layer 220 on the bottomof the first trench 231 may be an anisotropic dry etching process.

In one embodiment, the process of etching the source and drain materiallayer 220 on the bottom of the first trench 231 may use the interfacelayer 210 as an etching stop layer.

It should be noted that in one embodiment, the interface layer 210 maybe optionally formed on the surface of the substrate 200 before thesource and drain material layer 220 is formed. Therefore, the interfacelayer 210 may be exposed on the bottom of the second trench 222.

By using the mask layer 230 and the protective layer 240 as a mask toetch the source and drain material layer 220 on the bottom of the firsttrench 231, the distances from the center of the second trench 222 tothe two sidewalls of the first trench 231 are consistent with eachother.

The first trench 231 may cover the second trench 222. The first trench231 may have a cross-section area larger than the second trench 222. Inone embodiment, projections of centers of the first and second trencheson a surface the substrate may be substantially overlapped.

In FIG. 8, the protective layer 240 (see FIG. 7) may be removed afterthe second trench 222 is formed.

The process for removing the protective layer 240 may be an etchingprocess, such as a dry etching process or a wet etching process.

In the process of removing the protective layer 240, the ratio of theetching rate between the protective layer 240 and the mask layer 230 maybe in a range from 3 to 10.

In the process of removing the protective layer 240, the ratio of theetching rate between the protective layer 240 and the mask layer 220 maybe in a range from 3 to 10.

A channel material layer and a gate structure on the channel materiallayer are formed after the protective layer 240 is removed. The channelmaterial layer may be located on the sidewalls and the bottom of thefirst trench 231 and the sidewalls and the bottom of the second trench222.

After the protective layer 240 is removed, the source and drain materiallayer 220, which is located on the two sides of the second trench 222and on the bottom of the first trench 231, may be exposed. And the sizesof the source and drain material layer 220, located on the two sides ofthe second trench 222 and on the bottom of the first trench 231, areconsistent with each other.

The method of forming the channel material layer and the gate structureis described below with reference to FIG. 9 and FIG. 10.

In FIG. 9, a channel material film 250 may be formed on the sidewallsand the bottom portion of the first trench 231, on the sidewalls and thebottom the second trench 222, and on the mask layer 230.

The channel material film 250 may further be formed on the surface ofthe interface layer 210.

The channel material film 250 may be used to form a channel materiallayer.

The material of the channel material film 250 may be MoS₂, and theprocess for forming the channel material film 250 may be a solid sourcechemical vapor deposition process.

The material of the subsequently formed channel material layer may be acrystalline material. The electrical conductivity of the crystallinestate material may be high, and the channel material layer may have agood conductive property.

The material of the channel material film 250 may be MoS₂, and thechannel material film 250 may be formed on the bottom of the secondtrench 222 by using a solid source chemical vapor deposition process.The channel material film 250 on the bottom of the second trench 222 maybe used as a channel material layer on the bottom of the second trench222.

In addition, when using MoS₂ as the material for the channel materialfilm 250, the channel material film 250 may be ultra-thin.

The solid source chemical vapor deposition process includes: taking asolid as a generating source; evaporating and sublimating the solidsource; and forming a channel material film 250 with decompositionreactions in the atmosphere of the carrier gas.

When the channel material film 250 is made of MoS₂, the parameters ofthe solid source chemical vapor deposition process may include: a solidsource including MoO₃ and S; a carrier gas including Ar; and atemperature in a range of approximately 600° C.-900° C., such as 755° C.

The thickness of the channel material film 250 may be in a range ofapproximately 5 nm-10 nm, such as about 7 nm. Thickness of thesubsequently formed channel material layer may be in a range ofapproximately 5 nm-10 nm. The present method may be used to form asemiconductor device with an ultra-thin channel.

The thickness of the channel material film 250 may be in a range from 5nm to 10 nm. If the channel material film 250 is thinner than 5 nm, itwill be too difficult to control the process; and if the channelmaterial film 250 is thicker than 10 nm, the subsequently formed channelmaterial layer may be too thick, and it will be too hard to improve thecontrol capability of the gate structure on the channel.

As illustrated in FIG. 9, a gate structure material layer 260 may beformed on the surface of the channel material film 250.

The gate structure material layer 260 may include: a gate dielectricmaterial layer (not shown) on the surface of the channel material film250; and a gate electrode material layer (not shown) on the surface ofgate dielectric material layer.

The material of the gate dielectric material layer may be a high-k (kmay be greater than 3.9) dielectric material.

The gate electrode material layer may be made of metal, such as copperand/or tungsten.

The process for forming the gate dielectric material layer may be adeposition process, such as an atomic layer deposition process, a plasmachemical vapor deposition process, a low-pressure chemical vapordeposition process, or a sub-atmospheric chemical vapor depositionprocess.

The process for forming the gate electrode material layer may be adeposition process, such as a sputtering process or a plasma chemicalvapor deposition process.

In FIG. 10, a filling layer 270 may be formed to fill in the firsttrench 231 (see FIG. 9) and the second trench 222 (see FIG. 9) after thegate structure material layer 260 is formed.

The material of the filling layer 270 may be made of silicon oxide or abottom anti-reflective coating (B ARC) material.

The process for forming the filling layer 270 may include a spin coatingprocess.

As shown in FIG. 11, the gate structure material layer 260 and thechannel material film 250 are planarized until the top surface of themask layer 230 and the top surface of the filling layer 270 are exposed.Then the channel material film 250 forms a channel material layer 251,and the gate structure material layer 260 forms a gate structure 261.

The process for planarizing the gate structure material layer 260 andthe channel material film 250 may be a chemical mechanical grindingprocess.

The channel material layer 251 may be made of MoS₂.

The channel material layer 251 may be located not only on the sidewallsand the bottom of the second trench 222, but also on the bottom of thefirst trench 231. As a result, the effective length of the channelmaterial layer 251 may be relatively big, and the distance between thesource and drain doped regions on the two sides of the subsequent gatestructure may be relatively big correspondingly. And the short channeleffect of the semiconductor device may be improved.

The channel material layer 251 may be made of MoS₂. The advantages ofusing MoS₂ may include that the semiconductor device may be providedwith an ultra-thin channel and the channel material layer 251 may beformed on the surface of the interface layer 210.

The thickness of the channel material layer 251 may be in a range ofapproximately 5 nm-10 nm.

The gate structure 261 may include a gate dielectric layer located onthe channel material layer 251 and a gate electrode layer on the gatedielectric layer.

The gate dielectric layer corresponds to the gate dielectric materiallayer, and the gate electrode layer corresponds to the gate electrodematerial layer.

In FIG. 12, after the gate structure material layer 260 and the channelmaterial film 250 are planarized, the filling layer 270 (see FIG. 11)may be removed.

In one embodiment, the method may further include removing the masklayer 230 (see FIG. 11) after forming a channel material layer 251 and agate structure 261.

In one embodiment, the mask layer 230 may be removed in the process ofremoving the filler layer 270. The filling layer 270 and the mask layer230 may be removed separately, in another embodiment.

In one embodiment, the filling layer 270 and the mask layer 230 may beremoved by an etching process.

As shown in FIG. 13, after the mask layer 230 is removed, a source anddrain doped region 280 may be formed on the two sides of the gatestructure 261 in the source and drain material layer 220.

For example, source and drain ions may be injected into the source anddrain material layer 220 on the two sides of the gate structure 261 byadopting an ion implantation process and a source and drain doped region280 may be formed on the two sides of the gate structure 261 in thesource and drain material layer 220.

After the channel material layer 251 and the gate structure 261 areformed, the size of the channel material layer 251 on the source anddrain material layer 220 on one side of the second trench 222 may beconsistent with that of the channel material layer 251 on the other sideof the second trench 222. And the size of the gate structure 261 on thesource and drain material layer 220 on one side of the second trench 222may also be consistent with the size of the gate structure 261 on theother side of the second trench 222. Therefore, the sizes of the sourceand drain material layer 220 on the two sides of the gate structure 261are consistent with each other, and the size of the source and draindoped region 280 may also be consistent. The self-aligned metal gatewith channel substantially reduces the size difference of the source anddrain material layer on the two sides of the gate structure 261.Therefore, the stability of the electrical performance of thesemiconductor device may be improved.

As such, the source region and the drain region are formed in the sourceand drain material layer respectively on sides of the channel materiallayer. In one embodiment, projections of centers of the gate structureand the channel material layer on a surface the substrate may besubstantially overlapped.

Correspondingly, the embodiment further provides a semiconductor deviceformed by the above discussed method. Comparing with the conventionaltechniques, the present disclosure has the following advantages.According to the forming method of the semiconductor device provided bypresent disclosure, the channel material layer is formed along thesidewalls and the bottom of the first trench and of the second trench,and an ultra-thin channel material layer is formed. The ultra-thinchannel material layer limits the channel size of the semiconductordevice, and the semiconductor device is provided with an ultra-thinchannel. The channel material layer is located not only on the sidewallsand the bottom of the second trench, but also on the bottom of the firsttrench. Therefore, the effective length of the channel material layer isrelatively large, and the short channel effect of the semiconductordevice is further improved. Then a source and drain doped region isformed on the two sides of the gate structure in the source and drainmaterial layer. A protective layer is formed on the sidewall of thefirst trench, and then the mask layer and the protective layer are usedjointly to define the position of the second trench. The mask layer andthe protective layer are used as a mask to etch the source and drainmaterial layer on the bottom of the first trench, and the distances fromthe center of the second trench to the two sidewalls of the first trenchare consistent with each other. The source and drain material layer,which is located on the two sides of the second trench and on the bottomof the first trench, is exposed after the protective layer is removed.The size of the source and drain material layer located on the two sidesof the second trench and on the bottom of the first trench is alsoconsistent with each other. After the channel material layer and thegate structure are formed, and the size of the channel material layer onthe source/drain material layer on one side of the second trench isconsistent with that of the channel material layer on the other side ofthe second trench. Similarly, the size of the gate structure on thesource/drain material layer on one side of the second trench isconsistent with that of the gate structure on the other side of thesecond trench. Therefore, the sizes of the source and drain materiallayer on the two sides of the gate structure are consistent with eachother, and the stability of the electrical performance of thesemiconductor device is improved.

The embodiments disclosed herein are exemplary only. Other applications,advantages, alternations, modifications, or equivalents to the disclosedembodiments are obvious to those skilled in the art and are intended tobe encompassed within the scope of the present disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a source and drain material layer formed on the substrate, wherein thesource and drain material layer contains a first trench extendingthere-through; a channel material layer, including a U-shape portionconformally formed on a bottom and sidewalls of the first trench and twoL-shape portions formed on a top surface of the source and drainmaterial layer, wherein the two L-shape portions connects with two endsof the U-shape portion, respectively, and the two L-shape portions forma second trench, and each of the two L-shape portions of the channelmaterial layer includes: a first planar portion levelled on the topsurface of the source and drain material layer, and a second planarportion directly on the top surface of the source and drain materiallayer, perpendicular to the first planar portion, and extending in adirection perpendicular to the top surface of the source and drainmaterial layer; a source region and a drain region formed in the sourceand drain material layer respectively on sides of the channel materiallayer, wherein the second planar portion has a planar surface coplanarwith a side surface of the source region and the drain region, and anundoped portion of the source and drain material layer is locatedbetween the source region and the channel material layer and anotherundoped portion of the source and drain material layer is locatedbetween the drain region and the channel material layer; and a gatestructure conformally formed on the channel material layer, on thebottom and the sidewalls of each of the first trench and the secondtrench, wherein the gate structure has a recess and the recess has asymmetrical step structure.
 2. The semiconductor device according toclaim 1, wherein: projections of centers of the gate structure and thechannel material layer on a surface the substrate are substantiallyoverlapped.
 3. The semiconductor device according to claim 1, furtherincluding: an interface layer formed between a surface of the substrateand the source and drain material layer, and the first trench exposes asurface portion of the interface layer.
 4. The semiconductor deviceaccording to claim 1, wherein: the channel material layer is made of amaterial comprising MoS₂.
 5. The semiconductor device according to claim1, wherein: a thickness of the channel material layer is in a range ofapproximately 5 nm-10 nm.
 6. The semiconductor device according to claim1, wherein the gate structure layer comprises: a gate dielectric layeron the channel material layer; and a gate electrode layer on the gatedielectric layer.
 7. The semiconductor device according to claim 1,further including: an interface layer on a surface of the substrate. 8.The semiconductor device according to claim 7, wherein: the source anddrain material layer is on a top surface of the interface layer, and thefirst trench exposes a portion of the top surface of the interface layerand the interface layer closes off the first trench at the top surfaceof the interface layer.